编写一个简单的发布者和订阅者(C++)
设置环境变量
source /opt/ros/foxy/setup.bash
创建功能包
ros2 pkg create --build-type ament_cmake cpp_package --dependencies rclcpp std_msgs
创建发布者
cd ~/ros2_ws/src/cpp_package/src
touch publisher_member_function.cpp
将下面c++代码复制publisher_member_function.cpp文件中
#include <chrono>
#include <functional>
#include <memory>
#include <string>
#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
using namespace std::chrono_literals;
/* This example creates a subclass of Node and uses std::bind() to register a
* member function as a callback from the timer. */
class MinimalPublisher : public rclcpp::Node
{
public:
MinimalPublisher()
: Node("minimal_publisher"), count_(0)
{
publisher_ = this->create_publisher<std_msgs::msg::String>("topic", 10);
timer_ = this->create_wall_timer(
500ms, std::bind(&MinimalPublisher::timer_callback, this));
}
private:
void timer_callback()
{
auto message = std_msgs::msg::String();
message.data = "Hello, world! " + std::to_string(count_++);
RCLCPP_INFO(this->get_logger(), "Publishing: '%s'", message.data.c_str());
publisher_->publish(message);
}
rclcpp::TimerBase::SharedPtr timer_;
rclcpp::Publisher<std_msgs::msg::String>::SharedPtr publisher_;
size_t count_;
};
int main(int argc, char * argv[])
{
rclcpp::init(argc, argv);
rclcpp::spin(std::make_shared<MinimalPublisher>());
rclcpp::shutdown();
return 0;
}
对上述代码进行简单分析：
头文件部分
#include <chrono>
#include <functional>
#include <memory>
#include <string>
#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
命名空间
using namespace std::chrono_literals;
使用面向对象中的继承，类名为MinimalPublisher继承rclcpp::Node这个类，继承方式为public;
class MinimalPublisher : public rclcpp::Node
私有成员有回调函数timer_callback，计时器对象timer_,发布者对象publisher_,统计数量的count_;
该timer_callback函数是设置消息数据和实际发布消息的地方。该RCLCPP_INFO宏确保每个发布的消息都打印到控制台。还有计时器、发布者和计数器字段的声明
private:
void timer_callback()
{
auto message = std_msgs::msg::String();
message.data = "Hello, world! " + std::to_string(count_++);
RCLCPP_INFO(this->get_logger(), "Publishing: '%s'", message.data.c_str());
publisher_->publish(message);
}
rclcpp::TimerBase::SharedPtr timer_;
rclcpp::Publisher<std_msgs::msg::String>::SharedPtr publisher_;
size_t count_;
公有成员:类的构造函数用于对私有成员进行初始化；
公共构造函数命名节点minimal_publisher并初始化count_为 0。在构造函数内部，发布者使用String消息类型、主题名称topic和所需的队列大小进行初始化，以在发生备份时限制消息。接下来，timer_被初始化，这会导致timer_callback函数每秒执行两次。this指的是该节点
public:
MinimalPublisher()
: Node("minimal_publisher"), count_(0)
{
publisher_ = this->create_publisher<std_msgs::msg::String>("topic", 10);
timer_ = this->create_wall_timer(
500ms, std::bind(&MinimalPublisher::timer_callback, this));
}
主函数部分
rclcpp::init初始化 ROS 2，并rclcpp::spin开始处理来自节点的数据，包括来自计时器的回调。
int main(int argc, char * argv[])
{
rclcpp::init(argc, argv);
rclcpp::spin(std::make_shared<MinimalPublisher>());
rclcpp::shutdown();
return 0;
}
ROS2相比ROS1在语法上有了很大的改变，在编程思路还是一样。
创建订阅者
cd ~/dev_ws/src/cpp_package/src
touch publisher_member_function.cpp
将下面c++代码复制subscriber_member_function.cpp文件中
#include <memory>
#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
using std::placeholders::_1;
class MinimalSubscriber : public rclcpp::Node
{
public:
MinimalSubscriber()
: Node("minimal_subscriber")
{
subscription_ = this->create_subscription<std_msgs::msg::String>(
"topic", 10, std::bind(&MinimalSubscriber::topic_callback, this, _1));
}
private:
void topic_callback(const std_msgs::msg::String::SharedPtr msg) const
{
RCLCPP_INFO(this->get_logger(), "I heard: '%s'", msg->data.c_str());
}
rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription_;
};
int main(int argc, char * argv[])
{
rclcpp::init(argc, argv);
rclcpp::spin(std::make_shared<MinimalSubscriber>());
rclcpp::shutdown();
return 0;
}
对上述代码进行简单分析：
头文件部分：
#include <memory>
#include "rclcpp/rclcpp.hpp"
#include "std_msgs/msg/string.hpp"
和发布者代码一样，定义一个类MinimalSubscriber继承rclcpp::Node
class MinimalSubscriber : public rclcpp::Node
私有成员：定义回调函数topic_callback将接收的消息打印到终端，订阅者对象subscription_；
private:
void topic_callback(const std_msgs::msg::String::SharedPtr msg) const
{
RCLCPP_INFO(this->get_logger(), "I heard: '%s'", msg->data.c_str());
}
rclcpp::Subscription<std_msgs::msg::String>::SharedPtr subscription_;
公有成员：与类名一样的构造函数对私有成员初始化，这里节点名是唯一标识，除非在不同的命名空间下可以有相同的节点名。this指的是该节点。
public:
MinimalSubscriber()
: Node("minimal_subscriber")
{
subscription_ = this->create_subscription<std_msgs::msg::String>(
"topic", 10, std::bind(&MinimalSubscriber::topic_callback, this, _1));
}
主函数部分和发布者相差不大。
修改CMakeLists.txt文件，将下面内容复制到文件中并保存。
add_executable(talker src/publisher_member_function.cpp)
ament_target_dependencies(talker rclcpp std_msgs)
add_executable(listener src/subscriber_member_function.cpp)
ament_target_dependencies(listener rclcpp std_msgs)
install(TARGETS
talker
listener
DESTINATION lib/${PROJECT_NAME})
编译
cd ~/ros2_ws/
colcon build --packages-select cpp_pubsub
. install/setup.bash
运行
启动发布者talker节点
ros2 run cpp_pubsub talker
结果：
启动订阅者listener节点
ros2 run cpp_pubsub listener
结果：
